Description of problem:

Running `openstack tripleo validator run --group some-group' returns a stack trace if no validation is present in "some-group". For instance:

openstack tripleo validator run --group post-overcloud-prepare
Exception occured while running the command
Traceback (most recent call last):
  File "/usr/lib/python3.6/site-packages/tripleoclient/command.py", line 32, in run
    super(Command, self).run(parsed_args)
  File "/usr/lib/python3.6/site-packages/osc_lib/command/command.py", line 41, in run
    return super(Command, self).run(parsed_args)
  File "/usr/lib/python3.6/site-packages/cliff/command.py", line 185, in run
    return_code = self.take_action(parsed_args) or 0
  File "/usr/lib/python3.6/site-packages/tripleoclient/v1/tripleo_validator.py", line 388, in take_action
    self._run_validator_run(parsed_args)
  File "/usr/lib/python3.6/site-packages/tripleoclient/v1/tripleo_validator.py", line 364, in _run_validator_run
    t.field_names = results[0].keys()
IndexError: list index out of range
list index out of range


Version-Release number of selected component (if applicable):
python3-validations-libs-1.0.5-1.20200914174429.4a660ec.el8ost.noarch

How reproducible:
Always

Steps to Reproduce:
1. deploy an undercloud
2. try running a group without any validation in it
3.

Actual results:
A stacktrace is shown

Expected results:
A message saying "no validation in that group" or something should be shown

Additional info:
Detected during downstream CI checks - thanks David R and Jad!